The behavioral life of a young mammal begins within a uterine environment, and makes an abrupt transition (via birth) to a markedly different postnatal environment. We propose a program of research designed to elucidate sensory experience in the rat during late fetal and early postnatal environment - the perinatal period (El5-P5) - as a key to the origins of behavioral organization. The proposed experiments test hypotheses of sensory development derived from adaptive problems faced by the fetus in the uterine environment, and from corresponding adaptational challenges encountered by the newborn in the postnatal neonatal niche, immediately after parturition. From this perspective, we propose to determine onsets of function for the tactile, chemical, vestibular, and thermal senses. Sensory function will be measured by heartrate and behavioral responses. Based on analyses of ontogenetic adaptations to their developmental niches, we hypothesize and test predicted patterns of inter-modal development of sensitivity, and intra-modal maturation in different body regions. Novel studies of behavior of pregnant and lactating rats will help identify kinds and amounts of stimulation to which offspring are normally exposed. These experiments should establish a theoretically-framed database on the psychobiology of fetal and neonatal life that will support additional studies of early behavioral structure and learning. The proposed research can make significant contributions to our basic understanding of neurobiological development, and to applied issues concerning care and management of premature and at-risk human infants.